Transmit power control plays an important role in interference-limited wireless communication systems, such as those based on Code Division Multiple Access (CDMA) technologies. In these systems, transmit power control allows a transmitter to transmit at sufficient power to achieve acceptable data error rates at the receiver over changing reception conditions, but prevents the transmitter from transmitting at excessive power to limit multi-user interference.
In particular, a receiver maintains an “inner-loop” and an “outer-loop” power control mechanism to provide power control feedback to a remote transmitter. The inner-loop generates this power control feedback in the form of Transmit Power Commands (TPCs). The TPCs instruct the transmitter to increase or decrease its transmit power depending on whether the estimated signal quality at the receiver is below or above a signal quality target (e.g., a target signal-to-interference ratio, SIR). Under changing signal propagation conditions, however, the outer-loop adjusts this signal quality target over time to achieve a given data error rate target (e.g., a block error rate, BLER).
Some contexts complicate the above approach to transmit power control. For example, commonly a transmitter transfers data to a receiver over one or more transport channels that are logically carried by a physical channel. Each transport channel permits transmission of different types or rates of data, depending on the use of one or more transport formats during a given transmission time interval, and specifies its own data error rate target. In this context, transmit power control varies the power on the physical channel to achieve all of the data error rate targets specified on the transport channels.
More particularly, with each transport channel specifying a data error rate target, the outer-loop maintains a signal quality target for each transport channel. The inner-loop then compares the estimated signal quality on the physical channel to a maximum of the signal quality targets. By generating TPCs based on this maximum signal quality target, the inner-loop ensures the power on the physical channel is sufficient to achieve even the most stringent of data error rate targets specified on the transport channels.
Yet the signal quality target maintained for a transport channel, needed to fulfill the data error rate target specified on that transport channel, depends on the used transport format. That is, because different transport formats specify different code block sizes, coding schemes, etc., different signal quality targets are required to achieve the same data error rate target. Thus, not only must the outer-loop continually adjust the signal quality target of a transport channel to account for changing signal propagation conditions, but the outer-loop must also adjust this target each time a transport channel switches to using a different transport format. Whether required because of changes in signal propagation conditions, transport format use, or both, rapid adjustment of a signal quality target is highly desired so that its value quickly converges to that needed to achieve the corresponding data error rate target.
Various prior control loop designs addressing this issue adjust a signal quality target according to a ‘jump’ algorithm. Per the jump algorithm, the outer-loop significantly increases a signal quality target when erroneous data is received. This significant increase facilitates a faster convergence time. On average, however, more correct data should be received than erroneous data. To achieve an unbiased average data error rate target, therefore, the outer-loop only slightly decreases the signal quality target when correct data is received. Yet because the outer-loop must adjust the signal quality target over multiple transmission time intervals, convergence of that target is still relatively slow.
Other prior control loop designs also experience relatively slow convergence. The control loop design described in U.S. Pat. No. 7,376,438 to Shiu et al., for example, maintains a signal quality target for each transport format. When a transport channel uses a given transport format, Shiu adjusts its corresponding signal quality target to account for changes in signal propagation conditions and generates TPCs based on this signal quality target. When the transport channel changes to using a different transport format, however, its corresponding signal quality target does not reflect current signal propagation conditions and must be adjusted accordingly. Thus, such a control loop design still experiences slow convergence over multiple transmission time intervals.